This invention relates to closures for sealing storage bore holes which constitute the final disposal site of radioactive waste and a method of applying the closures.
For disposing of highly radioactive waste obtained in the reprocessing of irradiated nuclear fuel, the waste is mixed with glass-forming materials and is melted to form a glass mass which is loaded in vessels made of a high-quality steel and is allowed to harden therein. The decay energy of the radioactive fission products is sufficient to heat the steel vessels beyond the ambient temperature during a period of approximately 30 to 50 years. Dependent upon the concentration and the age of the fission products, the generated initial temperatures may be several hundred degrees Centigrade. Governmental disposal projects in the Federal Republic of Germany provide for a final storage of such highly radioactive waste in rock salt formations after an intermediate storage of 5 to 10 years. For such a final disposal, the waste is introduced into vertical storage bore holes having a depth of 20 to 50 m. These bore holes have to be provided with an appropriate seal at the top.
Heretofore, essentially two methods have been suggested concerning the provision of such sealing closures:
(1) The highly radioactive waste is to be covered with ground salt. This method is noted in a paper entitled "Bericht uber das in der Bundesrepublik Deutschland geplante Entsorgungszentrum fur ausgediente Brennelemente aus Kernkraftwerken" (Report on the Proposed Disposal Center in the Federal Republic of Germany for Fuel Elements Used Up in Nuclear Power Plants), December 1976, page 86.
(2) A salt solution-resistant cement is poured over the highly radioactive waste. This method is noted in a dissertation by R. Proske, entitled "Beitrage zur Risikoanalyse eines hypothetischen Endlagers fur hochaktive Abfalle" (Contributions to the Risk Analysis of a Hypothetical Final Disposal Site for Highly Radioactive Waste), 1977, page 17.
The first method provides no hermetic closure if, as hypothetically presented in the report, water break-in occurs in the pit wall. In such a case the heat-generating waste would directly contact the salt solutions and the possibility of a contamination of the salt solutions by wash-out activity is not excluded. The heat sources induce a convection of the salt solutions which may lead to an entrainment of the radioactivity over wide areas.
If, as noted in the second method, the storage bore holes are sealed by cement, a number of problems remain unresolved. Thus, for example, upon pouring in the dough-like cement, perspiration water or excess water may contact the waste vessels as such water runs down the inner walls of the storage bore hole. This water is, by the .gamma.-radiation, decomposed radiologically among others, into H.sub.2 and O.sub.2 (oxyhydrogen). In addition, OH radicals and H.sub.2 O.sub.2 are formed which are strongly corrosive. Further, by the strong .gamma.-radiation, the water bound in the cement is also in part radiologically split, resulting in a radiation-caused damage to the cement. The radiation-resistance of the cement is approximately 10.sup.10 rad. Tests conducted with electrically heated sample waste vessels have shown that particularly the upper part of the storage bore holes undergo a significant constriction in cross section. The continuous contraction of the storage bore hole could conceivably affect the binding and hardening process of the cement to such an extent that a sufficient final strength of the closure arrangement is not obtained.